According to UK colour codes, a fire extinguisher that displays a black label contains which medium?

The CO2 (Carbon dioxide) fire extinguisher is red with a black label. It is suitable for Class B (fires involve flammable or combustible liquids, including petrol, grease, and oil) and Electrical fires. CO2 fire extinguishers are safe for both high and low electrical voltages and are the main type used in computer server rooms. Do not use CO2 for: Cooking oil/chip pan fires, Organic materials fires (Class A), Flammable metal fires. Be aware that CO2 fire extinguishers produce fumes that can asphyxiate if used in confined spaces, therefore the area should be ventilated as soon as the fire has been controlled.

A fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user (i.e., no escape route, smoke, explosion hazard, etc.), or otherwise requires the expertise of a fire brigade. Typically, a fire extinguisher consists of a hand-held cylindrical pressure vessel containing an agent that can be discharged to extinguish a fire.